Extractive distillation of tetrafluoroethylene with an oxygenated hydrocarbon



United States Patent 3,282,801 EXTRACTIVE DISTILLATION 0F TETRAFLUORO- ETHYLENE WITH AN OXYGENATED HYDRO- CARBON Herbert Albert Wiist, Vienna, W. Va., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Filed Feb. 6, 1963, Ser. No. 256,520

6 Claims. (Cl. 20360) The process of the present invention relates to the separation of fluorocarbons, and, more particularly, to the extractive distillation of tetrailuoroethylene.

One of the preferred methods of preparing tetrafiuoroethylene is by pyrolysis of chlorodifluoromethane at temperatures above 700 C., as disclosed in greater detail in US. 2,551,573, issued to Downing et al. on May 8, 1951. The Downing et a1. method results in the production of a large number of fluorocarbons in addition to tetrafluoroethylene. These compounds include hexafluoropropylene, perfluorocyclobutane, trifluorometihane, difluoromet'hane, chlorotrifluoromethane, perfluoroisobutylene, dichlorodifluoromethane, chlorotrifluoroethylene, dichlorotetrafiuoroethane, chlorotetrafluoroethane, chlorohexafluoropropane, a homologous series of compounds (11 up to 13) and a number of other compounds which have not been completely identified. In another method, tetrafluoroethylene is produced by pyrolysis of brornodifluoromethane. Similar to the pyrolysis of the chlorine analog, the pyrolysis of the bromodifi-uoromethane produces a large number of fluorocarbons in addition to tetrafluoroethylene. As in the case of the chlorodifiuoromethane, the p-yrolysate contains a number of compounds which form azeotropes with tetrafluoroethylene or boil so close to tetrafluoroe-thylene that separation by direct distillation is difiicult.

It is, therefore, one of the objects of the present invention to separate tetrafluoroethylene from fluorocarbons by R OH, R2OR3, R OOR5, and RsC\ where R to R are saturated aliphatic or aromatic hydrocarbon radicals, which are liquidat distillation conditions, and, in particular, those which contain up to 12 carbon atoms. The process of the present invention is based on the discovery that the volatility of fiuorocarbons boiling close to tetrafluoroethylene, which generally have from 1 to 2 carbon atoms, or form azeotropes with tetrafiuoroethylene, is so changed by the addition of the extractive distillation agents of the present invention, that the separation of these groups of compounds becomes a matter of simple distillation. Additionally, it was surprisingly found that these oxygenated hydrocarbons do not react with the reactive fluorocarbons separated by the process of the present invention, such as tetra-fiuoro ethylene.

Examples of azeotropes of perfluorocarbons and fluorocarbons separated by the extractive distillation of the present invention are tetrafluoroethylene and difluoromethane, and tetrafiuoroethylene and trifluoromethane. Most of the close boiling fluorocarbon and perfluorocarbon compounds will 'be apparent from any table listing the physical properties of fluorocarbon and perfluorocarbon compounds, an example of which is pentafluoroethane and tetrafluoroethylene.

The process of the present invention is of particular utility in combination with processes for the preparation of tetrafluoroethylene, which is prepared by the pyrolysis of chlorodifiuoromethane, bromodifluoromethane or by inorganic and organic [fluoride arc processes known in the art.

The addition of the above-described extractive distillation agents was found to rearrange the volatilities of the interfering fiu'orocarbons in such a manner as to greatly enhance the volatility of the tetrafluoroethylene, thereby preventing the formation of the undesirable azeotropes and also preventing the distillation of close boiling fluorocarbons. The prefer-red extractive distillation agents of the present invention are volatile hydrocarbon alcohols, ethers, esters and ketones which are liquid under distillation conditions and include methanol, ethanol, propanol, isopropanol, butanol, isobutano-l, tert-butyl alcohol, decanol, phenol, benzyl alcohol, o-cres-ol, dimethyl ether, diethyl ether, methyl ethyl ether, methyl propyl ether, methyl phenyl ether, ethyl benzyl ether, acetone, methyl ethyl ketone, diethyl ketone, acetophenone, propi'oplaenone, methyl acetate, ethyl formate, methyl propionate, ethyl acetate, propyl formate, etc. The quantity of the extractive distillation agent to be employed can be varied over a wide range. Even the addition of small quantities of the extractive distillation agent will significantly affect the volatility. In order to readily facilitate the separation of the higher boiling fiuoroinated hydrocarbons obtained from the pyrolysis, after removal of starting material and tetrafluoroethylene it is preferred to employ from 1 to 25 times the weight of the extractive distillation agent as compared to the weight of the higher boiling fluorocarbons.

Relative volatilities at atmospheric pressure of fluorocarbons forming azeotropes with tetrafluoroethylene are shown in Table I using mixtures comprising 3 parts of the indicated extractive distillation agent and 1 part of the tillation agent on relative volatility at atmospheric pressure is shown in Table II. Tetrafluoroethylene is again taken as the most volatile component.

TABLE II Wt. of acetone/wt. of fluorocarbon: Difluoromethane Relative volatilities of a tetrafiuoroethylene, fluoroform, difiuoromethane mixture were determined using acetone as the extractive distillation agent. The mixture contained 64 parts of tetrafiuoroethylene, 26 parts of fluoroforrn and 10 parts of difluoromethane. The ratio of acetone to the fluorocarbon mixture was 1:1.4. The relative volatilities were determined at a pressure of 125 p.s.i.a. It was determined that tetrafluoroethylene was 2.66 times as volatile as fluoroform and 3.78 times as volatile as difluoromethane.

The relative volatilities of the components of the mixture illustrated in the tables below were determined using acetone as the extractive distillation agent. In the determination illustrated in Table III, a pressure of 125 p.s.i.a. and a temperature of 63.5 C. were employed. The mole percentage of acetone in the liquid was 79.0 and the mole percentage of acetone in the vapor was 10.3. In the determination illustrated in Table IV, a pressure of 60.5 p.s.i.a. and a temperature of 768 C. were employed. The mole percentage of the acetone'in the liquid was 93.0 and the mole percentage of the acetone in the vapor was 15.9.

TABLE III M01 Percent Component Relative Volatility 1 Liquid Vapor Tetrafluoroethylene 11. 14 31. 53 Chloropentafluoroethane 5. 48 9. 60 1. 62 Hexafluoropropylene. 2. 86 3. 56 2. 27 Trifluoromethane. 19. 23. 71 2. 27 Pentafluoroethane 20. 26 12. 24 4. 70 Chlorodifluoromethane 33. 08 10. 95 8. 50

1 Tetrafluoroethylene is taken as the most volatile component.

1 Tetrafluoroethylene taken as the most volatile component.

Example I Crude tetrafluoroethylene, having the following composition.

Mol percent Tetrafluoroethylene 95 Pentafluoroethane 1 Difluoromethane 4 was fed into a 43" long, 1" diameter packed column (35-40 plates) operating at atmospheric pressure, 25" up from the base at a rate of 200 g./hr. Methanol at 70 C. was fed from the top of the column at 400 g./hr. Reflux was at an approximate ratio of 1:1. Temperature of the column ranged from +20 C. at the base to -76 C. at the top of the column. Analysis of the overhead showed the product to be greater than 99.90% pure tetrafluoroethylene. The same results are obtainedwhen a bromodifluoromethane pyrolysate containing 5 to 20 mol percent of difluoromethane and 0.1 to 0.2 mol per cent of entafluoroethane is employed.

4 Example I1 Crude tetrafluoroethylene, having the following composition Mol percent was fed vapor phase into a 43" long, 1" diameter packed column (35 40 theoretical plates) operating at atmospheric pressure, 25" up from the base at a rate of 400 g./hr. Acetone at 70 C. was fed from the top of the column at 600 g./ hr. Reflux was at an approximate ratio of 1:1. Temperature of the column ranged from 76 C. at the top of the column to +31 C. at the base. Purity of the tetrafiuoroethylene removed overhead was greater than 99.9 mol percent.

The process of the present invention provides a simple, effective and economic method for the recovery of pure tetrafluoroethylene when formed in admixture With other fiuorocarbons, such asare obtained in the pyrolysis of chlorodifluorornethan'e and bromodifiuoromethane.

I claim:

1. A process for separating tetrafiuoroethylene from fiuorocarbons containing from 1 to 2 carbon atoms and, in addition to fluorine, elements selected from the class consisting of hydrogen and halogen, having atomic numbers from 17 to 53 inclusive in admixture with the tetrafluoroethylene, the steps of distilling the mixture, by extractive distillation in the presence of an oxygenated hydrocarbon, liquid at distillation conditions and selected from the class consisting of alcohols, ethers, ketones and esters, said oxygenated hydrocarbons having the general formulas R OH, R OR R COR and R6COOR7, where R to R are radicals selected from the class consisting of saturated aliphatic and aromatic hydrocarbon radicals, and removing the tetrafiuoroethylene as an overhead from the distillation zone.

2. The process of claim 1 wherein the extractive dis- References Cited by the Examiner UNITED STATES PATENTS 2,339,160 1/1944 Dunn et al 20367 X 2,357,028 8/1944 Shires et al 260-652 2,456,184 12/ 1948 Greenwald 260652 2,473,911 6/ 1949 Sarsfield 260653 X 2,604,439 7/1952 Nixon 20362 X 3,101,304 8/1963 Wiist 20367 3,152,051 10/1964 Fainberg et al 20367 X NORMAN YUDKOFF, Primary Examiner.

WILBUR L. BASCOMB, JR., Examiner.

F. E. DRUMMOND, Assistant Examiner. 

1. A PROCESS FOR SEPARATING TETRAFLUOROETHYLENE FROM FLUOROCARBONS CONTAINING FROM 1 TO 2 CARBON ATOMS AND, IN ADDITION TO FLUORINE, ELEMENTS SELECTED FROM THE CLASS CONSISTING OF HYDROGEN AND HALOGEN, HAVING ATOMIC NUMBERS FROM 17 TO 53 INCLUSIVE IN ADMIXTURE WITH THE TETRAFLUOROETHYLENE, THE STEPS OF DISTILLING THE MIXTURE, BY EXTRACTIVE DISTILLATION IN THE PRESENCE OF AN OXYGENATED HYDROCARBON, LIQUID AT DISTILLATION CONDITIONS AND SELLECTED FROM THE CLASS CONSISTING OF ALCOHOLS, ETHERS, KETONES AND ESTERS, SAID OXYGENATED HYDROCARBONS HAVING THE GENERAL FORMULAS R1OH, R2OR3, R4COR5 AND R6COOR7, WHERE R1 TO R7 ARE RADICALS SELECTED FROM THE CLASS CONSISTING OF SATURATED ALIPHATIC AND AROMATIC HYDROCARBON RADICALS, AND REMOVING THE TERTRAFLUOROETHYLENE AS AN OVERHEAD FROM THE DISTILLATION ZONE. 